1. Field of the Invention
This invention pertains to high speed data transmission systems using multiple carriers and more particularly to the art of distributing the power and bits across various carriers so as to maximize the net data rate.
2. Related Art
A multi-carrier transmission system is one that employs more than one carrier for transmission of data using Frequency Division Multiplexing (FDM). Generally, to transmit a given bit stream over a multi-carrier transmission system, the bit stream is converted into parallel data. In a Quadrature Amplitude Modulation system, the parallel data may be used to choose a constellation point (a QAM symbol). These constellation points may be scaled by the fraction of the power they are allocated. The scaled constellation points are then modulated, i.e., converted from frequency to time domain signal and are transmitted over a channel such as a telephone line. The bit rate (number of bits allocated to each carrier) and the fraction of total power allocated to each carrier are variables in such a system. The number of bits per carrier determines the size of the constellation for that carrier. The fractional power is the portion of total power for all carriers which is allocated to any given carrier.
A receiver in a multi-carrier system demodulates the received signal to decode the transmitted constellation point. A constellation decoder on the receiver side converts these constellation points into bits, and the bits of all of the carriers are converted into a single bit stream.
The channel may impair the transmitted signal and the bits received on each sub channel may be in error. The bit error rate for a given carrier depends on the ratio of signal power to noise power, which is known as the Signal to Noise Ratio (SNR) for the carrier and the size of the constellation on that carrier. It is known that the aggregate bit rate is maximized when the parameters of bits per carrier and fraction of the power allocated to each carrier are appropriately chosen. In particular, the bit error rate on each of the carriers should be the same for the bit rate to be maximum given a target bit error rate. See, for example, John Bingham, “Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come,” IEEE Communications Magazine May 1990.
In general, it may not be possible to get a bit allocation at nominal power that will ensure that all carriers operate at the same bit error rate. One way of achieving uniform bit error rate across all carriers is to boost or buck the signal power, i.e., in effect increase or decrease the SNR and control the bits allocated in each carrier. See, for example, “Method For Improving Modem Performance By Controlling Transmitted Power Of Modem And Modem Implementing The Same,” U.S. Pat. No. 5,265,151.
For each of the carriers, if the SNR is known to the receiver, then the receiver can determine, with an appropriate algorithm, parameters such as bits per carrier and the fraction of the power allocated to that carrier. These parameters can then be communicated to the transmitter using a suitable messaging scheme. These parameters are chosen subject to constraints on maximum power per carrier, minimum power per carrier and the total power. It is therefore necessary for the receiver to estimate the signal to noise ratio before it can decide on the bit and power allocation. The transmitter during the initial phase of establishing connection sends known bits (known constellation points) so that the receiver can estimate the signal to noise ratio. Using the signal to noise ratio as computed from the known constellation points, the receiver decides on the bit and power allocation to each carrier.
One solution to the allocation problem is to compute all feasible allocations and choose the best allocation scheme. Though such a solution works in theory, it is not suitable from an implementation point of view. This is due to the fact that the number of feasible solutions are exponential in the number of carriers and the number of steps in bit allocation per carrier possible without violating power constraints. Several sub optimal algorithms are known. See, for example, Peter S. Chow, John M. Cioffi and John A. C. Bingham, “A Practical Discrete Multitone Transceiver Loading Algorithm for Data Transmission Over Spectrally Shaped Channels”, IEEE Transactions on Communications Vol. 43 1995; R. H. Fischer and J. B. Huber, “A New Loading Algorithm for Discrete Multitone Transmission”, IEEE Globecom, 1996. These algorithms are also complex from an implementation point of view.
Also, the SNR obtained on a carrier may not be sufficient for carrying even the smallest bit rate constellation (2Bmin point constellation) at the target bit error rate. One possible solution would be to increase the transmission power on that carrier and thus increase the SNR. However, increasing transmission power may not be always possible due to constraints on total power and the maximum power allowed per carrier. In such cases, one way to use that carrier for data transmission is to add redundancy to reduce the bit error rate. Fractional bit transmission is one such method of adding redundancy to increase data rate given a target bit error rate.
In fractional bit transmission, sets of carriers that individually cannot carry one or more bits are collectively used for data transmission. All carriers in a set carry the same QAM symbol. In effect each carrier carries a fraction of a symbol, hence the name fractional bit allocation. The objective being to maximize the data rate at a given target bit error rate, smaller the n higher the data rate/per carrier and lesser is the power per bit. A drawback of fractional bit transmission is an excessive Peak to Average Ratio (PAR).
The bit and power allocation are decided by the receiver and are communicated to the transmitter. The protocol for communicating integral bit allocations is simple. As an example, the protocol indicates the number of bits and relative boost or reduction in power for each carrier. See, ANSI T1.413-1995, American National Standard for Telecommunications—Network and Customer Installation Interfaces—Asymmetric Digital Subscriber Line (ADSL) Metallic Interface, August 1995. The message conveying fractional bit allocation should convey the fractional allocation on each carrier and also indicate the carriers that carry the same symbol. This allocation message must be as small as possible to reduce overhead.
One protocol known for conveying fractional allocation is as follows: For each carrier carrying fractional bit, the offset of the nearest carrier that carries the same bit is indicated. A bit field (say m bits) is allocated for conveying the nearest carrier index. This protocol has the following limitations. First, two carriers separated by distance greater the 2m−1 cannot carry the same symbol. Second, the message length is directly proportional to the allowable index difference between the carriers that carry the same bit. See Yuri Goldstein, “Parallel Transmission to Increase Reliable Data Rate in a DMT-based System”, ITU-T Standardization sector SG-15, NG-039, August 1999.
Accordingly, there is a need for a FDM transmission system capable of the fractional bit allocation that is able to control the maximum number of carriers carrying the same symbol. Furthermore, the fractional bit allocation should be able to control the number of symbols that are transmitted using fractional bit allocation. These features are desirable from the point of view of controlling the Peak-to-Average Ratio (PAR) of the signal.
In fractional bit transmission, the same symbol is sent on more than one carrier. This increases the PAR of the signal. A methods for mitigating the increase in the PAR of the signal is also needed.